docs: def add_formdata(self, content=None):
          # type: (Optional[Dict[str, str]]) -> None
          """Add data as a multipart form-data request to the request.
  
          We only deal with file-like objects or strings at this point.
          The requests is not yet streamed.
  
          :param dict headers: Any headers to add to the request.
          :param dict content: Dictionary of the fields of the formdata.
          """
          if content is None:
              content = {}
          content_type = self.headers.pop('Content-Type', None) if self.headers else None
  
          if content_type and content_type.lower() == 'application/x-www-form-urlencoded':
              # Do NOT use "add_content" that assumes input is JSON
              self.data = {f: d for f, d in content.items() if d is not None}
          else: # Assume "multipart/form-data"
              self.files = {f: self._format_data(d) for f, d in content.items() if d is not None}